bitcoin/src/allocators.h

// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2013 The Bitcoin developers
// Distributed under the MIT/X11 software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#ifndef BITCOIN_ALLOCATORS_H
#define BITCOIN_ALLOCATORS_H

#include <map>
#include <string>
#include <string.h>

#include <boost/thread/mutex.hpp>
#include <boost/thread/once.hpp>
#include <openssl/crypto.h> // for OPENSSL_cleanse()

/**
 * Thread-safe class to keep track of locked (ie, non-swappable) memory pages.
 *
 * Memory locks do not stack, that is, pages which have been locked several times by calls to mlock()
 * will be unlocked by a single call to munlock(). This can result in keying material ending up in swap when
 * those functions are used naively. This class simulates stacking memory locks by keeping a counter per page.
 *
 * @note By using a map from each page base address to lock count, this class is optimized for
 * small objects that span up to a few pages, mostly smaller than a page. To support large allocations,
 * something like an interval tree would be the preferred data structure.
 */
template <class Locker> class LockedPageManagerBase
{
public:
    LockedPageManagerBase(size_t page_size):
        page_size(page_size)
    {
        // Determine bitmask for extracting page from address
        assert(!(page_size & (page_size-1))); // size must be power of two
        page_mask = ~(page_size - 1);
    }

    ~LockedPageManagerBase()
    {
        assert(this->GetLockedPageCount() == 0);
    }


    // For all pages in affected range, increase lock count
    void LockRange(void *p, size_t size)
    {
        boost::mutex::scoped_lock lock(mutex);
        if(!size) return;
        const size_t base_addr = reinterpret_cast<size_t>(p);
        const size_t start_page = base_addr & page_mask;
        const size_t end_page = (base_addr + size - 1) & page_mask;
        for(size_t page = start_page; page <= end_page; page += page_size)
        {
            Histogram::iterator it = histogram.find(page);
            if(it == histogram.end()) // Newly locked page
            {
                locker.Lock(reinterpret_cast<void*>(page), page_size);
                histogram.insert(std::make_pair(page, 1));
            }
            else // Page was already locked; increase counter
            {
                it->second += 1;
            }
        }
    }

    // For all pages in affected range, decrease lock count
    void UnlockRange(void *p, size_t size)
    {
        boost::mutex::scoped_lock lock(mutex);
        if(!size) return;
        const size_t base_addr = reinterpret_cast<size_t>(p);
        const size_t start_page = base_addr & page_mask;
        const size_t end_page = (base_addr + size - 1) & page_mask;
        for(size_t page = start_page; page <= end_page; page += page_size)
        {
            Histogram::iterator it = histogram.find(page);
            assert(it != histogram.end()); // Cannot unlock an area that was not locked
            // Decrease counter for page, when it is zero, the page will be unlocked
            it->second -= 1;
            if(it->second == 0) // Nothing on the page anymore that keeps it locked
            {
                // Unlock page and remove the count from histogram
                locker.Unlock(reinterpret_cast<void*>(page), page_size);
                histogram.erase(it);
            }
        }
    }

    // Get number of locked pages for diagnostics
    int GetLockedPageCount()
    {
        boost::mutex::scoped_lock lock(mutex);
        return histogram.size();
    }

private:
    Locker locker;
    boost::mutex mutex;
    size_t page_size, page_mask;
    // map of page base address to lock count
    typedef std::map<size_t,int> Histogram;
    Histogram histogram;
};


/**
 * OS-dependent memory page locking/unlocking.
 * Defined as policy class to make stubbing for test possible.
 */
class MemoryPageLocker
{
public:
    /** Lock memory pages.
     * addr and len must be a multiple of the system page size
     */
    bool Lock(const void *addr, size_t len);
    /** Unlock memory pages.
     * addr and len must be a multiple of the system page size
     */
    bool Unlock(const void *addr, size_t len);
};

/**
 * Singleton class to keep track of locked (ie, non-swappable) memory pages, for use in
 * std::allocator templates.
 *
 * Some implementations of the STL allocate memory in some constructors (i.e., see
 * MSVC's vector<T> implementation where it allocates 1 byte of memory in the allocator.)
 * Due to the unpredictable order of static initializers, we have to make sure the
 * LockedPageManager instance exists before any other STL-based objects that use
 * secure_allocator are created. So instead of having LockedPageManager also be
 * static-intialized, it is created on demand.
 */
class LockedPageManager: public LockedPageManagerBase<MemoryPageLocker>
{
public:
    static LockedPageManager& Instance() 
    {
        boost::call_once(LockedPageManager::CreateInstance, LockedPageManager::init_flag);
        return *LockedPageManager::_instance;
    }

private:
    LockedPageManager();

    static void CreateInstance()
    {
        // Using a local static instance guarantees that the object is initialized
        // when it's first needed and also deinitialized after all objects that use
        // it are done with it.  I can think of one unlikely scenario where we may
        // have a static deinitialization order/problem, but the check in
        // LockedPageManagerBase's destructor helps us detect if that ever happens.
        static LockedPageManager instance;
        LockedPageManager::_instance = &instance;
    }

    static LockedPageManager* _instance;
    static boost::once_flag init_flag;
};

//
// Functions for directly locking/unlocking memory objects.
// Intended for non-dynamically allocated structures.
//
template<typename T> void LockObject(const T &t) {
    LockedPageManager::Instance().LockRange((void*)(&t), sizeof(T));
}

template<typename T> void UnlockObject(const T &t) {
    OPENSSL_cleanse((void*)(&t), sizeof(T));
    LockedPageManager::Instance().UnlockRange((void*)(&t), sizeof(T));
}

//
// Allocator that locks its contents from being paged
// out of memory and clears its contents before deletion.
//
template<typename T>
struct secure_allocator : public std::allocator<T>
{
    // MSVC8 default copy constructor is broken
    typedef std::allocator<T> base;
    typedef typename base::size_type size_type;
    typedef typename base::difference_type  difference_type;
    typedef typename base::pointer pointer;
    typedef typename base::const_pointer const_pointer;
    typedef typename base::reference reference;
    typedef typename base::const_reference const_reference;
    typedef typename base::value_type value_type;
    secure_allocator() throw() {}
    secure_allocator(const secure_allocator& a) throw() : base(a) {}
    template <typename U>
    secure_allocator(const secure_allocator<U>& a) throw() : base(a) {}
    ~secure_allocator() throw() {}
    template<typename _Other> struct rebind
    { typedef secure_allocator<_Other> other; };

    T* allocate(std::size_t n, const void *hint = 0)
    {
        T *p;
        p = std::allocator<T>::allocate(n, hint);
        if (p != NULL)
            LockedPageManager::Instance().LockRange(p, sizeof(T) * n);
        return p;
    }

    void deallocate(T* p, std::size_t n)
    {
        if (p != NULL)
        {
            OPENSSL_cleanse(p, sizeof(T) * n);
            LockedPageManager::Instance().UnlockRange(p, sizeof(T) * n);
        }
        std::allocator<T>::deallocate(p, n);
    }
};


//
// Allocator that clears its contents before deletion.
//
template<typename T>
struct zero_after_free_allocator : public std::allocator<T>
{
    // MSVC8 default copy constructor is broken
    typedef std::allocator<T> base;
    typedef typename base::size_type size_type;
    typedef typename base::difference_type  difference_type;
    typedef typename base::pointer pointer;
    typedef typename base::const_pointer const_pointer;
    typedef typename base::reference reference;
    typedef typename base::const_reference const_reference;
    typedef typename base::value_type value_type;
    zero_after_free_allocator() throw() {}
    zero_after_free_allocator(const zero_after_free_allocator& a) throw() : base(a) {}
    template <typename U>
    zero_after_free_allocator(const zero_after_free_allocator<U>& a) throw() : base(a) {}
    ~zero_after_free_allocator() throw() {}
    template<typename _Other> struct rebind
    { typedef zero_after_free_allocator<_Other> other; };

    void deallocate(T* p, std::size_t n)
    {
        if (p != NULL)
            OPENSSL_cleanse(p, sizeof(T) * n);
        std::allocator<T>::deallocate(p, n);
    }
};

// This is exactly like std::string, but with a custom allocator.
typedef std::basic_string<char, std::char_traits<char>, secure_allocator<char> > SecureString;

#endif